The man did not realize until too late that he had started his trip when it was very dangerous because of the low temperature. The narrator states that it is seventy-five degrees below zero. The dog, "a big native husky," knows instinctively that it is too cold to be traveling. This is one of the reasons it is uneasy or nervous.
But the brute had its instinct. It experienced a vague but menacing apprehension that subdued it and made it slink along at the man's heels, and that made it question eagerly every unwonted movement of the man as if expecting him to go into camp or to seek shelter somewhere and build a fire.
There is an extreme danger of breaking through the snow into a stream of running water and getting wet. This would necessitate stopping to build a fire immediately in order to dry his clothing and avoid severe injury from frost bite. After a few close calls, the man forces the dog to go on ahead of him. This is another reason the dog is uneasy or nervous.
Once again, however, he had a close call; and once, suspecting danger, he compelled the dog to go on in front. The dog did not want to go. It hung back until the man shoved it forward, and then it went quickly across the white, unbroken surface.
When the man finally finds himself in truly desperate trouble because he can no longer build a fire with his frozen hands, he gets the idea of killing his dog and cutting it open so that he can warm his hands inside the dog's body. But the dog doesn't trust him. They do not have a friendly relationship. The dog is only motivated by fear of his brutal owner and dependence on him for food. Jack London does a remarkable job of describing the dog's feelings and instincts.
Something was the matter, and its suspicious nature sensed danger; it knew not what danger but somewhere, somehow, in its brain arose an apprehension of the man.
This is a third reason why the dog felt uneasy or nervous. It relies on its instincts. But its instincts are always correct; whereas the man, who relies on reason, is always making serious mistakes. His biggest mistake was coming out on this journey in such cold weather. He had been warned against it by old-timers, but he has a stubborn and independent disposition. He realizes in the end that he should have listened. The man dies but the dog survives. After it realizes that its master is dead, it leaves him.
Then it turned and trotted up the trail in the direction of the camp it knew, where were the other food-providers and fire-providers.